1. Field of the Invention
This invention relates generally to a speed change lever assembly which is used for displacing a front or rear deraileur relative to a multiple sprocket. More particularly, the present invention concerns a speed change lever assembly of the type wherein pressing of one lever causes the deraileur to move in one direction, whereas pressing of another lever causes the deraileur to move in the opposite direction.
2. Description of the Prior Art
As is well known, a bicycle speed change system comprises a deraileur mounted adjacent a front or rear gear (multiple sprocket) for movement axially of the gear, and a lever assembly mounted on a suitable portion of a bicycle frame or handlebar and connected to the deraileur by means of a control cable. The lever assembly is operated to displace the deraileur relative to the gear, thereby shifting the chain from one sprocket to another of the gear for speed change.
A typical speed change lever assembly includes a single lever pivotally supported at its boss portion on a fixed shaft. The boss portion of the lever works also as a cable winder connected to one end of a control cable whose other end is connected to a deraileur. When the lever is pivoted in one direction, the control cable is wound up on the lever boss portion against a tension applied to the cable by a return spring which is incorporated in the deraileur. When the lever is pivoted in the opposite direction, the cable is paid out from the lever boss portion under the tension of the return spring. Such cable movements are transmitted to the deraileur for movement thereof relative to a front or rear gear, thereby performing an intended chain shift.
With the lever assembly described above, the lever has to be pressed and pulled respectively for winding up and paying out the cable. Therefore, it is necessary for the cyclist to use two different kinds of finger movements for operating the lever in the two pivotal directions. Thus, the operability of the typical lever assembly is relatively poor. Such poor operability becomes particularly problematic when the lever provides a large pivotal angle.
In an attempt to solve the above problem, there has been proposed to use a double lever assembly, as disclosed for example in Japanese Patent Publication No. 55-20909 (Published: June 5, 1980; Applicant: Bridgestone Cycle Co., Ltd.; Inventor: Kunitoshi KOSAKAI) or Japanese Utility Model Publication No. 59-43191 (Published: Dec. 20, 1984; Applicant: Shimano Industrial Co., Ltd.; Inventor: Keizo SHIMANO). Specifically, the double lever assembly comprises a pair of levers pivotally supported on a common pivot shaft respectively on both sides of a fixed plate. The fixed plate has a retaining hole for holding a ball whose diameter is slightly larger than the wall thickness of the fixed plate, so that the ball is partially projectable from the retaining hole on either side of the plate. One of the levers is connected to a control cable under a tension, and formed with a positioning hole which disengages from the ball in a first pivotal limit position (cable pay-out position) of the one lever but engages with the ball in a second pivotal limit position (cable winding position) of the one lever. The other lever is also formed with a positioning hole which engages with the ball in a first pivotal limit position of the other lever but disengages from the ball in a second pivotal limit position of the other lever. Both levers are always urged toward their respective first pivotal limit positions by the tension of the control cable and/or a separate spring.
In operation, when the one lever assumes its first limit position with the positioning hole thereof disengaging from the ball, the other lever is held in its first position by the ball engaging with the positioning hole of the other lever. In this condition, the control cable is fully paid out from the one lever. When the one lever is pivoted to its second limit position to fully wind up the control cable, the ball comes into engagement with the positioning hole of the one lever but disengages from the positioning hole of the other lever, so that the other lever is pivoted by the urging force of the spring to its second limit position which substantially corresponds to the first limit position of the one lever. On the other hand, when the other lever is pressed to its first limit position, the ball comes into engagement with the positioning hole of the other lever lever but disengages from the positioning hole of the one lever, so that the one lever is pivoted to its first limit position by the tension of the control cable (additionally by the urging force of the spring).
In this way, the double lever assembly is capable of shifting up and down only by pressing either one of the two levers. However, such a lever assembly is disadvantageous in the following respects.
First, each of the levers can take only two limit positions, so that the prior art double lever assembly is not capable of selecting three or more speeds. Second, the pivotal movement (returning movement) of the one lever from its second position to its first position is initiated only after the other lever has been completely pivoted to its first position. Thus, the pivotal pressing operation of the other lever does not cause the control cable to be immediately paid out from the one lever. In other words, the prior art double lever assembly fails to provide a quick operating response at least with respect to the cable pay-out operation.